A variety of different types of mass spectrometers or analyzers are known. These include quadrupole mass analyzers, time of flight (TOF) mass analyzers, ion cyclotron resonators, and ion trap mass spectrometers (IT-MS). One type of mass spectrometer of recent interest is a linear ion trap (LIT) Fourier-transform mass-spectrometry (FT-MS) system.
In an LIT FT-MS system, mono-energetic ions are reflected back and forth between a pair of electrostatic (electrode) mirrors. An inductively coupled pick-up coil records the ion current as a function of time. Fourier analysis of this signal current yields a spectrum of the ion oscillation frequencies, which is directly related to the mass spectrum of the ions in the trap. Useful signal is obtained only when each different ion mass species remains separately and tightly bunched.
Zajfman et al. U.S. Pat. No. 6,744,042, the contents of which are incorporated herein in their entirety, describes such a system in which the dynamics induced by the proper choice of the electrostatic mirror potentials in conjunction with the coulomb repulsion between the constituent ions generates an effective self-bunching force, causing the ions to reside in self-sustaining spatially-limited ion packets that can propagate for long periods of time, allowing high-resolution measurements to be made of the mass spectrum of the trapped ions.
However, an unintentional effect of this self-bunching force for identical ions is that ions that are not of identical mass-to-charge ratio, but are adequately close in both mass and spatial position, may experience a net attractive self-bunching force as well. This can cause inaccuracies in the bunch constituent identities as well as a possible merging of similar but non-identical bunches. These effects, which are referred to herein as Ion Bunch Coalescence (IBC), can limit the accuracy, resolution, and sensitivity of the LIT FT-MS.
What is needed, therefore, is a method of operating a LIT FT-MS system that can reduce or eliminate IBC. What is also needed is an LIT FT-MS system that exhibits reduced IBC.